Von Economo and Fork Neurons across Three Independently Evolved Vocal-Learning Bird Lineages

Abstract

Independently evolved instances of vocal learning in birds — in parrots (Psittaciformes), oscine songbirds (Passeriformes), and hummingbirds (Trochiliformes) — offer a natural experiment for testing whether complex behavioural convergence is accompanied by convergence at the level of individual neuron morphology. This study used comparative Golgi impregnation to characterize von Economo neuron (VEN)-like cells and the related fork neuron across the vocal control nuclei of representative species from all three lineages: the Indian ringneck parrot (Psittacula krameri), the zebra finch (Taeniopygia guttata), the canary (Serinus canaria), and Anna's hummingbird (Calypte anna). VEN-like neurons were identified in every vocal control nucleus examined in all four species. Despite the species spanning close to an order of magnitude in brain mass, key morphological ratios were remarkably conserved: soma aspect ratio ranged only from 1.33 to 1.37, soma volume relative to adjacent non-VEN neurons ranged from 3.8× to 4.8×, and dendritic arbor reduction relative to pyramidal-type neurons ranged from 58% to 65%. Fork neurons co-occurred with VEN-like neurons in every nucleus and species examined at a strikingly stable ratio of 2.2:1 (range 2.0:1–2.3:1), with the highest combined densities consistently located in the HVC/NLC and RA equivalents. These findings indicate that VEN-like and fork neuron morphology is not a species-specific idiosyncrasy but a conserved cellular solution that has arisen independently at least three times within the avian vocal-learning lineages examined, paralleling similar convergent cellular specializations previously documented across vocal-learning mammals.